Combining 2 UV maps into one
This is the simplest way I know of merging UV maps in Blender:


Separate all the polygons of the head to a new object.
Delete the 'body' UV map from the head object.
Rename the 'head' UV map of the head object to 'body'.
Delete the 'head' UV map of the body object as you don't need it.
Merge the head object back onto the body and all the UVs will now be in the 'body' UV map.